In general, CMP is used to planarize individual layers (e.g., dielectric or metal layers) during integrated circuit (IC) fabrication on a semiconductor wafer. CMP removes undesirable topographical features of the IC on the wafer. For example, CMP removes metal deposits subsequent to damascene processes, and excess oxide from shallow trench isolation steps. Similarly, CMP may also be used to planarize inter-metal dielectrics (IMD), or devices with complex architecture, such as system-on-a-chip (SoC) designs and vertical gate structures (e.g., FinFET) with varying pattern density.
CMP utilizes a reactive liquid medium, commonly referred to as a slurry, and a polishing pad to provide chemical and mechanical control to achieve planarity. Either the liquid or the polishing pad may contain nano-size inorganic particles to enhance chemical reactivity and/or mechanical activity of the CMP process. The pad is typically made of a rigid, micro-porous polyurethane material capable of performing several functions including slurry transport, distribution of applied pressure across a wafer, and removal of reacted products. During CMP, the chemical interaction of the slurry forms a chemically modified layer at the polishing surface. Simultaneously, the abrasives in the slurry mechanically interact with the chemically modified layer, resulting in material removal. The material removal rate in a CMP process is related to slurry abrasive concentration and the average coefficient of friction (f) in the pad/slurry/wafer interfacial region. The extent of normal & shear forces during CMP and f typically depends on pad tribology. Recent studies indicate that pad material compliance, pad contact area, and the extent of lubricity of the system play roles during CMP processes. See, e.g, A. Philiposian and S. Olsen, Jpn. J. Appl. Phys., vol. 42, pp 6371-63791; Chemical-Mechanical Planarization of Semiconductors, M. R. Oliver (Ed.), Springer Series in Material Science, vol. 69, 2004; and S. Olsen, M. S. Thesis, University of Arizona, Tuscon, Ariz., 2002.
An effective CMP process not only provides a high polishing rate, but also a finished (e.g., lacking small-scale roughness) and flat (e.g., lacking in large-scale topography) substrate surface. The polishing rate, finish, and flatness are thought to be governed by the pad & slurry combination, pad/wafer relative velocity, and the applied normal force pressing the substrate against the pad.
Two commonly occurring CMP non-uniformities are edge effects and center slow effects. Edge effects occur when the substrate edge and substrate center are polished at different rates. Center slow effects occur when there is under-polishing at the center of the substrate. These non-uniform polishing effects reduce overall flatness.
Another commonly observed problem relates to slurry transport and distribution. In the past, polishing pads had perforations. These perforations, when filled, distributed slurry when the pad was compressed. See, e.g., J. Levert et al, Proc. Of the International Tribology Conf., Yokohoma, 1995. This method was ineffective because there was no way to directly channel the excess slurry to where it was most needed (i.e., at the wafer surface). Currently, macro-texturing of pads is typically done through ex-situ pad surface groove design. See, e.g., U.S. Pat. Nos. 5,842,910; 5,921,855; 5,690,540; and T. K. Doy et al, J. of Electrochem. Soc., vol. 151, no. 3, G196-G199, 2004. Such designs include, circular grooves (e.g., concentric grooves referred to as “K-grooves”) and cross-hatched patterns (e.g., X-Y, hexagons, triangles, etc.). The groove profile may also be rectangular with ‘V-’, ‘U-’ or saw-tooth shaped cross sections.